Casting process for producing hermetically sealed housings for hard disk drives

ABSTRACT

An aluminum alloy casting having a globular microstructure forms a housing for a hard disk drive by pouring a molten aluminum alloy with a fine grain and a non-dendritic structure into the shot chamber defined by a shot sleeve of a vertical die cast press. Heat is removed from the molten alloy to produce a semi-solid slurry which is forced upwardly into a die cavity through an offset elongated funnel gate opening positioned to received and merge hotter slurry within the center portion of the shot chamber with cooler slurry adjacent the shot sleeve. The solidified alloy forms a housing that has a gas leak rate that meets the requirements of being hermetically sealed.

RELATED APPLICATION

This application claims the benefit of Provisional Patent Application Ser. No. 61/838,512, filed Jun. 24, 2013.

BACKGROUND OF THE INVENTION

Aluminum alloy components such as the base housing of a hermetically sealed hard disk drive require gas pressure tightness. The aluminum alloy components are commonly cast using a conventional high pressure die casting process that results in porous castings. These castings then require a sealant impregnation process to seal them for gas pressure tightness. The reliability of these components produced in this manner can be limited. The current processes when used to manufacture parts with relatively complex geometries generally yield components with shrink porosity and micro-porosity which limits their use in gas pressure tight applications. Hermetically sealed Hard Disk Drives (HDD) charged with helium are required for the next generation of data recording technology. They are necessary for future increases in track density that yield greater disk storage capacity and for lower power consumption.

It is desirable to provide a method of casting metals that can yield components that offer gas pressure tightness with extremely small gas leakage rates without the need of sealant impregnation

SUMMARY OF THE INVENTION

This invention defines a method and apparatus for a semi-solid molding process (SSM) that is ideally suited to the needs of producing castings for hermetic applications. Unlike conventional die castings, die cast parts produced using semi-solid molding processes can be produced substantially free of porosity. Because of the thixotropic nature of the semi-solid slurry and non-turbulent way that it flows into a casting die, the process is capable of producing cast parts having thin sections, geometric complexity and close dimensional tolerances without entrapped porosity. These are the characteristics required for many cast components used in gas and fluid pressure tight applications. The present invention enables pressure tight cast components to be produced using the semi-solid method and apparatus as described in this patent application. These components would not require sealant impregnation to be pressure tight.

What is required in the thixotropic aluminum slurry is a uniform primary aluminum panicle size in the range of 50 to 80 microns. A uniform distribution of this microstructure is required throughout the injected aluminum volume. The method and apparatus of this invention meets these requirements through the steps described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1, 1A & 1B show a typical Hard Disk Drive (HDD) housing which is approximately 5.75″×3.88″×1.0″ high and has thin walls of about 0.12″ or less for weight savings;

FIG. 2 shows a cross-section through a mold set and a portion of a vertical die cast press used to inject aluminum slurry into the die cavity for the HDD housing;

FIG. 3 shows the micro-structure of the SSM non-dendritic structure within the range of 50 to 80 microns;

FIG. 4 a perspective section of a 3-D model of the mold set of FIG. 2 to show more clearly the gating geometry;

FIG. 5 is a photo of the “biscuit” of solidified aluminum remaining after the casting is made and showing the offset funnel gate; and

FIG. 6 is an exploded diagram of FIG. 2 and showing the temperature profile within the semi-solid slurry prior to injection.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

Referring to FIG. 2, an upper die or mold section 16 with ejector pins 21 is secured to the bottom of the adapter plate 17 which is attached to a vertical die casting hydraulic cylinder of a vertical die cast machine, as disclosed in U.S. Pat. No. 6,901,991 which issued to applicant, and the disclosure of which is herein incorporated by reference. A lower die or mold section 15 is attached to a gate plate 14 and is fixed to the lower platen of the vertical die cast machine. The gate plate 14 defines an offset funnel gate passage 19 that cooperates with the axially symmetric tapered nozzle opening 20 in the lower mold 15. The die cavity 18 defines the geometry of the cast housing 30 shown in FIG. 1. A shot sleeve 11 defines a cylindrical shot chamber which receives a corresponding shot piston 10. The shot piston 10 has a water cooled cavity 22 to control the shot piston temperature.

In operation of the vertical die cast machine according to the process disclosed in above mentioned U.S. Pat. No. 6,901,991, a molten aluminum alloy is poured into the vertical shot chamber defined by the shot sleeve 11 and shot piston 10 where the alloy cools to form the semi-solid slurry 12. The slurry is then injected vertically upwardly into the die cavity 18 by way of the offset funnel gate 19 and tapered nozzle 20. The mold sections open to allow the cast housing 30 to be ejected from the die cavity 18 in the upper die or mold 16.

In forming the housing 30, rapid heat removal occurs as the molten aluminum is poured at 650 deg. C into the vertical shot chamber. The aluminum volume is cooled by the water cooled shot piston and shot sleeve. This leads to the generation of primary aluminum nucleation sites within the aluminum volume. Greater cooling occurs near the shot sleeve surface. This leads to a non-uniform temperature and nuclei distribution within the aluminum volume. The aluminum is hotter near the center of the aluminum volume in the shot chamber as shown by the temperature profile in FIG. 6.

During the injection of the aluminum into the mold cavity, it is forced through an offset funnel gate 19 within the gate plate 14. The purpose of this gate geometry is to cause forced convection of heat from the hotter aluminum near the center of the aluminum volume with the cooler aluminum adjacent or closer to the shot sleeve surface. This forced convection or heat advection disperses the nuclei in the injected volume and promotes the non-dendritic growth of the primary aluminum particles. Temperature gradients within the injected volume result in the non-dendritic or globular morphology of the primary aluminum particles. The shot piston moves vertically upward at about 1.8 inches per second. The aluminum surface rises within the offset funnel gate 19 at the entrance at about 35 inches per second at the bottom of the gate opening. At the entrance to the tapered gate nozzle 20 in the lower mold plate 15, the aluminum is flowing at about 180 inches per second. Turbulent flow within the semi-solid temperature range of the aluminum alloy is occurring within the offset funnel gate 19. This turbulence is a requirement for the thixotropic slurry to develop.

The aluminum then flows through the axially symmetric tapered nozzle 20 into the mold cavity 18. The purpose of this gate geometry is to promote a laminar flow of aluminum into the mold cavity. This gate geometry reduces the turbulence in the aluminum flow that occurs within the offset funnel gate. The aluminum flow rate as it enters the mold cavity is about 800 to 850 inches per second. This flow rate or gate velocity into the mold cavity is substantially higher than the gate velocity commonly used in the art of semi-solid molding which is about 200 inches per second. After the aluminum solidifies in the die cavity 18 to form the housing 30 for a hard disk drive, the housing or aluminum body has a hermeticity leak rate of about 5×10 e−8 cc/second which is required to obtain a housing 30 or other cast component or part that does riot require sealant impregnation to be pressure tight.

While the method and form of apparatus for semi-solid molding herein described constitutes a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise method and form of apparatus described, and that changes made therein without departing from the scope and spirit of the invention as defined in the appended claims. 

What is claimed is:
 1. A method of producing an aluminum alloy casting having a globular microstructure and adapted to form a hermetically sealed housing for a hard disk drive, the method comprising the steps of forming a molten aluminum alloy with a fine grain and a non-dendritic structure, directing the molten alloy into a shot chamber defined by a shot sleeve of a vertical die cast press, removing heat from the molten alloy within the shot chamber to produce a semi-solid slurry, and forcing the slurry into a die cavity through an offset funnel gate opening positioned to received and merge convection heat from hotter slurry within the center portion of the shot chamber with convection heat from a cooler slurry adjacent the shot sleeve.
 2. A method as defined in claim 1 and including the step of forcing the molten slurry from the funnel gate opening through a tapered nozzle opening within a lower mold section and extending to the die cavity.
 3. An aluminum alloy casting having a globular microstructure and adapted to form a hermetically sealed housing for a hard disk drive, said casting comprising a body of solidified semi-solid aluminum alloy, and said body having a hermeticity leak rate of about 5×10 e−8 cc/second.
 4. Apparatus for producing a metal part within a die cavity defined by a die set mounted on a vertical die cast press, said press comprising a shot sleeve defining a shot chamber having a generally vertical axis, a shot piston supported for axial movement within said shot sleeve, a coolant passage within said shot piston for cooling a molten metal within said shot chamber to within a predetermined temperature range, a power actuator for moving said shot piston upwardly to transfer molten metal from said shot chamber through a gate opening into the die cavity where the molten metal solidifies to form the metal part, and said gate opening comprising an offset funnel gate opening positioned to receive and merge hotter slurry within a center portion of said shot chamber with cooler slurry adjacent said shot sleeve.
 5. Apparatus as defined in claim 4 and including a mold plate above said offset funnel gate opening and defining a tapered nozzle opening extending to the die cavity. 